In producing a mass spectrum for a sample, particularly when methods such as electrospray or ion spray are used, there is commonly unwanted background noise. It is always desirable to have the signal level relatively high compared with the background, in order to be able to distinguish the signal. Many approaches, some elaborate and expensive, have been used to achieve this objective.
The inventors have found that when the sample is used to produce multiply charged ions (as commonly occurs in electrospray, ion spray and related techniques), the background consists mainly of singly charged ions. In this situation, using the techniques described below, it is possible to reduce the unwanted background considerably (i.e. to improve the S/N ratio).